Credit: © 2010 ACS

Liesegang rings are circular patterns formed by ions precipitating in well-defined areas, rather than a single place, of a gel. They are, however, formed by a relatively small number of combinations of species and supports. Most of the species are ionic and offer little chance to tune their properties such as size or solubility, so the rings are generally not controllable.

Now, Bartosz Grzybowski and colleagues from Northwestern University have used metallic nanoparticles covered with positively or negatively charged molecules to create more predictable Liesegang rings (pictured; J. Am. Chem. Soc. 132, 58–60; 2010). A sheet of agarose gel was soaked in a solution of similarly charged nanoparticles. A disc was then cut from the middle of the sheet and filled with a solution of the oppositely charged nanoparticles. As these diffuse through the gel over a day or more, they form rings of precipitate of equal amounts of nanoparticles of both polarities.

Although the nanoparticle system is similar to traditional Liesegang-type ones, they form by different mechanisms, with precipitation of ions governed by the solubility product of the salts, but nanoparticle aggregation works in a different way. Grzybowski and colleagues developed a model that uses diffusion and aggregation coefficients to predict the trends in spacing between the rings.

The original version of this story first appeared on the Research Highlights section of the Nature Chemistry website.